German patent 3,445,428 describes a glass sorting device wherein the scraps of glass fall through a chute. Thereby they fall through light barriers with one light source and a plurality of photoelectric detectors. The detectors are made sensitive to different wavelengths by means of filters . The signals of the detectors are integrated. Depending on the detector signals, effectors are energized. The effectors are strippers. The scraps of glass fall on a conveyor and are directed to different containers by the strippers.
European patent application 0,426,893 describes a method and a device for sorting scraps of glass, wherein the intensity of the light directed through the scraps of glass is measured at two different wavelengths. The difference of the intensities of the light transmitted at the two wavelengths serve as measuring data for characterizing the glass. A fraction of the scraps of glass is separated, with which the difference is smaller than a first theshold and the intensities are larger than a second threshold. Such scraps of glass are regarded as colorless or "white" glass. The thresholds represent classification parameters. A compressed air stream serves as effector.
Flaps serve as effectors in the German patent application 3,731,402. The colors of the scraps of glass are differentiated by means of light barriers with color filters.
A paper by Germer "Optoelektronischer Glasscherben-Sortierer" in "messen+prufen / automatik" (1983), 286-288 describes a sorting device, wherein the transmission is measured at two wavelengths in the green and red spectral ranges. The quotients of the transmissions serve as measuring data for characterizing the type of the glass.
In all these cases, classification parameters have to be fixed. The fixing of these classification parameters presents certain problems.
The fractions obtained by the sorting are the more valuable the clearer the various colors of the glass are separated. This is particularly true for colorless glass, where even small proportions of colored glass considerably reduce the value of the waste glass. Therefore the setting of the classification parameters is very critical.
On the other hand, the measuring data such as the quotient of the transmissions at two different wavelengths is, by no means uniform for a fraction of scraps of glass, for example of green glass. Within the fraction, the measuring data may ray between more or less wide limits. Furthermore, the measuring data may depend on other influences, for example on the contamination level, the ambient temperature or atmospheric humidity (moisture, frozen moisture or fogging of scraps of glass). Therefore the setting of the classification parameters is not simple.